1. Field of the Invention
The present invention relates to a novel ferritic steel having high strength and high toughness, and a method of producing the same.
The ferritic steel of the invention has high durability in corrosive or stress loading environments and is suited for use for the manufacture of power-generating turbine parts, nuclear fuel cladding pipes, automobile mufflers and so on.
2. Description of the Prior Art
Among the ferrous materials, ferritic steel has the advantage not found in austenitic steel that it is resistant to stress corrosion cracking and low in thermal expansion coefficient, so that it is widely used as a material of structural components.
In recent years, there has been an increasing rise of demand for higher performance and smaller weight of products, so that even higher strength of structural materials has been desired. The conventional techniques for strengthening structural materials such as quenching and tempering heat treatment, solid-solution strengthening by an addition of alloying elements and precipitation strengthening had the problem of their tendency to cause deterioration of toughness of the produced material, and low toughness of the material has been a serious restriction on product designing. Recently, the researchers have pursued studies in earnest on grain refinement strengthening known as a material strengthening technique which causes no deterioration of toughness, and now it is possible to obtain a steel material having ultrafine crystal grains with an average grain size of not greater than 1 xcexcm.
The powder metallurgy method adopting a mechanical grinding process such as mechanical alloying has made it possible to make large scale components, allowed enlargement of the degree of freedom of shaping after consolidation, and enabled refining of crystal grains to the nanometer order by mechanical pulverization, making it possible to obtain a high strength ultrafine grain structure with a grain size of several hundred nanometers depending on the consolidation process.
In order to obtain an ultrafine grain structure, it has been proposed and practiced to introduce dispersed particles which suppress the growth of crystal grains during consolidation. Carbides or oxides are used as dispersed particles, and one example using carbides is disclosed in JP-A-2000-96193. Also, examples using oxides are described in JP-A-2000-104140, JP-A-2000-17370 and JP-A-2000-17405.
JP-A-2000-17405 discloses a method of producing a high strength ultrafine grain steel containing SiO2, MnO, TiO2, Al2O3, Cr2O3, CaO, TaO and Y2O3. The role of the oxide-forming alloying elements is substantially defined to the supply of dispersed particles, and their amount is limited as excess precipitation results in a deterioration of toughness.
JP-A-2000-17370 describes a method of producing a high strength ultrafine grain steel directly from iron ore or iron sand by powder metallurgy method applying the mechanical alloying technique, and it states that since SiO2, Al2O3, CaO, MgO and TiO2 in the raw powder are refined by mechanical alloying or finely precipitated during consolidation, it is possible to control the growth of crystal grains while making harmless the otherwise adverse effect of the oxides on mechanical properties of the produced steel.
JP-A-2000-17370 teaches also that it is possible to improve properties by adding one or more elemental powders of Al, Cu, Cr, Hf, Mn, Mo, Nb, Ni, Ta, Ti, V, W and Zr during mechanical alloying, but it is silent on effective amounts of the powders to be added and the properties to be improved.
As the effect of grain refining on toughness, it is known that the ductile-brittle transition temperature (DBTT) is lowered by such refining, and it has been reported that DBTT could be made lower than the liquid nitrogen temperature in the steel material having its crystal grains refined by thermomechanical treatment employing rolling vis-à-vis the material produced by melting/casting. However, with the art of powder metallurgy, it is difficult to attain high toughness simply by refining of crystal grains due to the brittlement factors such as particle boundaries of a starting powder and dispersed particles. Herein, the term xe2x80x9cstarting powderxe2x80x9d means the powder produced by mechanical alloying.
An object of the present invention is to produce a ferritic steel having high strength and high toughness by powder metallurgy method making use of mechanical alloying techniques and to provide a novel ferritic steel having high strength and high toughness.
According to the present invention, at least one compound-forming element selected from the group consisting of Zr, Hf, Ti and V is added when producing a ferritic steel powder by mechanical alloying.
The compound-forming elements are combined with O, C and N originally contained in the ferritic steel powder or getting mixed therein from the atmosphere to form a carbide, an oxide and a nitride, respectively, in the course of consolidation of the ferritic steel powder produced by mechanical alloying. The formed compounds function as pinning particles for controlling the growth of crystal grains to improve toughness of the consolidated ferritic steel.
The invention ferritic steel contains, by weight, not more than 1% Si, not more than 1.25% Mn, 8 to 30% Cr, not more than 0.2% C, not more than 0.2% N, not more than 0.4% O, and a total amount of not more than 12% of at least one compound-forming element selected from the group consisting of Ti, Zr, Hf, V and Nb in amounts of not more than 3% Ti, not more than 6% Zr, not more than 10% Hf, not more than 1.0% V and not more than 2.0% Nb. It may optionally further contain not more than 3% Mo, not more than 4% W and not more than 6% Ni. The balance consists of Fe and unavoidable impurities. The invention ferritic steel has an average crystal grain size of not more than 1 xcexcm after consolidation.
The compound-forming element contained in the invention ferritic steel is preferably at least one selected from Ti, Zr and Hf, and it is particularly preferable that at least one of Ti, Zr and Hf be contained in amounts of not more than 3% Ti, not more than 6% Zr and not more than 10% Hf for a total amount of not more than 12%.
These compound-forming elements exist in the form of carbide, nitride and oxide in the consolidated ferritic steel.
The total content of O, C and N in the consolidated ferritic steel is a key factor for obtaining a ferritic steel having high strength and high toughness. It is desirable that the total content of O, C and N is not more than 66% by weight of the total content of Zr, Hf and Ti. In the case where Zr and Hf are contained as the compound-forming elements, the total content of O, C and N is preferably not more than 66% by weight of the total content of Zr and Hf.
According to the present invention, there are provided ferritic steels containing any one of Zr, Hf and Ti respectively as the compound-forming element, a ferritic steel containing all of Zr, Hf and Ti, a ferritic steel containing Zr and Hf, and a ferritic steel containing all of Zr, Hf, Ti, V and Nb.
The invention ferritic steel can be produced by encapsulating the steel powder produced by mechanical alloying, and subjecting the encapsulated steel powder to plastic deformation working.
The plastic deformation working is preferably carried out at a temperature of 700xc2x0 C. to 900xc2x0 C. The plastic deformation working can be effected by such a method of extrusion or hydrostatic pressing. Extrusion is preferably conducted in an extrusion ratio of 2 to 8, and hydrostatic pressing is preferably performed under a hydrostatic pressure of 190 MPa or higher. Preferably, hydrostatic pressing is followed by forging.
It is also desirable to conduct, after plastic deformation, a heat treatment for heating the work at 600xc2x0 C. to 900xc2x0 C. under a hydrostatic pressure of 10 to 1,000 MPa as this treatment contributes to the further enhancement of toughness.
In encapsulation of the steel powder produced by mechanical alloying, the capsules filled with the powder are preferably evacuated.
Before the encapsulation, the steel powder may be subjected to a heat treatment at a temperature from 200xc2x0 C. to lower than 700xc2x0 C. for 1 to 10 hours.
In the ferritic steel producing method of the invention, when the raw powders are mixed and subjected to mechanical alloying, the whole or part of at least one compound-forming element selected from Zr, Hf, Ti, V and Nb is preferably used in the form of an elemental powder and mixed with other alloy steel powders. Although the compound-forming elements of Zr, Hf, Ti, V and Nb may be used in the form of a compound, it is desirable to use an elemental powder of a compound-forming element(s) or a pre-alloyed powder containing a compound-forming element(s) when producing the mechanically alloyed ferritic steel.
The present inventors have revealed that when producing steel by the powder metallurgy method, gaseous substances of O (oxygen), C (carbon) and N (nitrogen) give a great influence to ductility and toughness of the product steel. The gaseous substances, beside those derived from the raw powders, include those brought in from the atmosphere during the course of mechanical pulverization of the raw powders. They may also be derived from the working tools. The excessive gaseous substances form non-metallic inclusions on the powder particle surfaces. Such non-metallic inclusions impair metal to metal bonding of the powders to greatly deteriorate ductility and toughness of the consolidated steel.
In the present invention, the gaseous substances of O, C and N are combined with the compound-forming elements such as Zr, Ti and Hf to form compounds which function as pinning particles for suppressing the crystal grain growth.
Herein below there will be provided a description on the metal structure, the chemical composition, and the production conditions in the present invention.
Cr is an element which serves for improving corrosion resistance of the invention steel, and is contained in an amount of preferably not less than 8 wt % in the steel. However, the Cr content should not exceed 30 wt % because the presence of the element in excess of 30 wt % may induce marked precipitation of the compounds which causes embrittlement of the product steel.
Zr, Hf and Ti combine with gaseous components of O, C and N to fix these, whereby the gaseous components are prevented to form non-metallic inclusions. Compounds between Zr, Hf or Ti, and O, C or N are very stable and finely dispersed in a matrix, and serve for pinning the grain boundary movement to suppress the crystal grain growth.
In the mechanical pulverizing process, inclusion of O and N from the atmosphere is unavoidable. Especially O is problematic as it exerts serious influence on the mechanical properties of the materials. Also, for the mechanical pulverizing process, it is necessary to use the working tools of a high strength material, for example, JIS SKD11 (AISI D2) or JIS SUJ2 (AISI 52100) with a high C content, which makes inclusion of C hardly avoidable.
The presence of free O, C and N included as impurities affects particle boundaries of the starting powder to cause embrittlement of the materials. Zr, Hf, and Ti act to inhibit the O, C and N from diffusing to particle boundaries of the starting powder and fix O, C and N in the form of oxides, carbides and nitrides in the powder, whereby they become the so-called pinning particles and contribute to suppression of growing of crystal grains, producing an effect of improving strength and toughness of the product steel.
The contents of Zr, Hf and Ti are mainly determined by the amounts of O, C and N after the mechanical pulverizing process. Inclusion of O, C and N during the mechanical pulverizing process can be suppressed to some extent by using a high-purity inert gas in gas atomization and mechanical pulverization processes. It is also effective to provide a coating on working tools such as balls for pulverization and/or the inner surface of a pulverization chamber prior to conducting the mechanical pulverizing process.
However, the amounts of the gaseous elements in the steel may be up to, by weight, 0.4% of O, 0.2% of C and 0.2% of N. Therefore, while their upper allowable limits are set at, by weight, 0.4% of O, 0.2% of C, and 0.2% of N, preferable contents are preferably 0.02 to 0.2% of O, preferably 0.002 to 0.15% of C and preferably 0.001 to 0.15% of N.
It is important to adjust the additive amounts of Zr, Hf and Ti so as to let the included elements O, C and N be quickly formed (precipitated) as Zr oxides (e.g. ZrO2), Hf oxides (e.g. HfO2), Ti oxides (e.g. TiO2), Zr carbides (e.g. ZrC), Hf carbides (e.g. HfC), Ti carbides (e.g. TiC), Zr nitrided (e.g. ZrN), Hf nitrided (e.g. HfN) or Ti nitrides (e.g. TiN) during heating at consolidating, and not to embrittle the steel.
Zr, Hf and Ti are added with their upper limits set at, by weight, 6% (preferably 0.01 to 4%) for Zr, 10% (preferably 0.01 to 8%) for Hf, and 3% (preferably 0.01 to 2.7%) for Ti. For reducing the amount of expensive Hf, it is desirable to add a small amount of Hf together with Zr. This is because usually Zr ores contain approximately 2 to 3 wt % of Hf. It is therefore expedient to add Hf in a proportional amount of not more than 3 wt %, preferably 0.01 to 2 wt % to that of Zr.
In case of adding Zr, Hf and Ti at the same time, in view of the probability that the extraneous elements O, C and N might be contained in maximum amounts of, by weight, 0.4% for O, 0.2% for C and 0.2% for N, and that the steel could be embrittled by the excessive precipitation of the compounds, it is preferable to add the said elements (Zr, Hf and Ti) in a total amount of up to 12% by weight (preferably 0.01 to 8% by weight).
In order to make the entered elements O, C and N harmless in the consolidation process, the total amount of Zr, Hf and Ti is adjusted so that the value provided by dividing the sum of absolute amounts of O, C and N by the sum of absolute amounts of Zr, Hf and Ti will become less than 66 wt %, preferably less than 38 wt %.
In case of adding Zr and Hf alone at the same time, it is also desirable that their total amount be adjusted so that the value provided by dividing the sum of absolute amounts of O, C and N by the sum of absolute amounts of Zr and Hf will become less than 35% by weight, preferably less than 17% by weight.
Mo, W, Ni, V and Nb may be added for the purpose of improving the functional and mechanical properties of the product steel for use in various environments.
Mo and W are usually dissolved in the matrix and partly precipitated as carbides to serve for strengthening the product material. It is therefore effective to add these elements for strengthening the product material. They are also useful for improving heat resistance of the material particularly when it is used at a high temperature. Excessive addition of either of these elements is undesirable as it tends to provoke precipitation of intermetallic compounds which becomes a cause of embrittlement of the product material. When adding Mo, it is added in an amount not exceeding 3% by weight, preferably 0.5 to 1.5% by weight, and when adding W, it is added in an amount not exceeding 4% by weight, preferably 0.5 to 3% by weight, more preferably 1.0 to 2.5% by weight.
Ni is also usually dissolved in the matrix and serves for improving corrosion resistance. Its presence is therefore effective for improving corrosion resistance of the product material. Its excessive addition, however, should be avoided as it unstabilizes the ferrite phase. When Ni is added, its amount added is preferably 0.3 to 1.0% by weight, with its upper limit being 6% by weight.
V and Nb, when added to a steel material, are usually precipitated as carbides to serve for strengthening the material. They also have an action to control the growth of crystal grains.
Excessive addition of these elements, however, causes embrittlement of the material. When V is added, its preferred amount range is not more than 1.0% by weight, especially 0.05 to 0.5% by weight, and when Nb is added, its preferred amount range is not more than 2.0% by weight, especially 0.2 to 1.0% by weight.
When two or more of the above-mentioned five elements Zr, Hf, Ti, V and Nb are added simultaneously, it is desirable that their total amount be adjusted to be not more than 12% by weight for the purpose of controlling excessive precipitation of the oxide, carbide and nitride. When their total amount exceeds 12% by weight, the rate of precipitation of the oxide, carbide and nitride elevates to cause embrittlement of the product material.
Si and Mn are added as a deoxidizer in production of the material powder, Mn being also useful as a desulfurizer. The content of Si should be not more than 1% by weight and the content of Mn should be not more than 1.25% by weight in conformity to the Japanese Industrial Standards (JIS) of ferritic stainless steel. However, in case of using the high-purity materials as the components and vacuum melting them to make a powder, it is not necessary to add Si and Mn.
The mechanically pulverized alloy powder is encapsulated in the metallic capsules and extruded at 700xc2x0 C. to 900xc2x0 C. in an extrusion ratio of 2 to 8 to produce a bulk material having high compactness and toughness while maintaining fine crystal grains.
When the extrusion temperature is below 700xc2x0 C., although the situation may vary depending on the extrusion ratio, there is a possibility to cause clogging, and also desired toughness may not be obtained due to accumulation of strain or other causes. The extrusion temperature, therefore, is preferably not lower than 700xc2x0 C. When it exceeds 900xc2x0 C., however, there may take place excessive growth of crystal grains, making it unable to obtain high strength of the product material. Therefore, the extrusion temperature is preferably 700xc2x0 C. to 900xc2x0 C.
When the extrusion ratio is less than 2, there may remain voids in the inside of the product material. On the other hand, when the extrusion ratio exceeds 8, separation tends to take place under the influence of fiber texture to lower toughness of the material. Clogging is also likely to occur. Thus, the preferred range of extrusion ratio is 2 to 8.
Even with the specimens which have been consolidated by giving plastic deformation to the powder to some extent, as in hot extrusion, after mechanical pulverization process, there are the occasions when the mechanical properties expected from the material structure can not be obtained under the restrictions of size and shape of the product or performance of the equipment. On such occasions, it is possible to improve toughness by a heat treatment under pressure of not lower than 10 MPa.
This is possible because, by the above heat treatment, the inter-particle connection is encouraged while controlling the growth of inter-particle compounds. When this heat treatment is conducted under a lower pressure, for example, under atmospheric pressure, the powder particle boundaries tend to become the compound-forming sites and may cause embrittlement of the product material.
Generally, the higher the pressure under which the heat treatment is conducted, the more desirable, but in view of the performance of the existing apparatus having a certain level of treating chamber capacity, the upper limit of pressure applicable is around 1,000 MPa. Therefore, pressure of the working atmosphere is preferably between 10 and 1,000 MPa.
In view of structural stability, it is desirable that the heat treatment be carried out basically at the consolidationtemperature or a lower temperature. For promoting inter-particle connection, the heat treatment is preferably carried out at a temperature not lower than 600xc2x0 C. Thus, the preferred range of heat treatment temperature is from 600xc2x0 C. to 900xc2x0 C.
Even in case of forming the pinning particles of the same composition, viz. the same type, it is possible to control the crystal grain size of the matrix according to the heating pattern in the consolidation process.
It is considered that in the powder after mechanical pulverization, the composing elements O, C and N of the pinning particles are either in a state of being dissolved in the matrix or exist as oxides, carbides and nitrides which are so fine that they can hardly function as the pinning particles.
If heating is conducted rapidly in this state, there is a tendency for the crystal grains to grow before the pinning particles are sufficiently precipitated or grown. It becomes easier to obtain a fine crystal structure by maintaining the temperature at which the pinning particles can form or grow lively before raising the temperature to the consolidation temperature.
In the case of the invention chemical composition, it is possible to confirm the presence of oxides, carbides and nitrides through an electron microscope by holding the composition at not lower than 200xc2x0 C. for one hour or more. When the composition is held at not lower than 700xc2x0 C. for more than 10 hours, many nonmetallic products are allowed to exist at the starting powder particle boundaries to impair toughness of the composition after consolidation. Therefore, the holding temperature before consolidation is preferably restricted to the range of 200xc2x0 C. to 700xc2x0 C., and the holding time is preferably 1 to 10 hours.
The mechanical properties of the ferritic steel obtained after consolidation are mostly dependent on the crystal grain size. According to the present invention, it is possible to obtain a structural strength surpassing 1,000 MPa while maintaining the same level of toughnessxe2x80x94about 1 MJ/m2 of Charpy impact valuexe2x80x94as the conventional ferritic steels.
It is hardly possible to obtain this level of strength and toughness with the conventional precipitation strengthening method, solid-solution strengthening method, heat treatment or powder metallurgy method.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.